Si₄O₄、n-pentane及Sr₂YRuO₆的第一性原理研究

【作者】 张萱；

【导师】 曾祥华；

【作者基本信息】 扬州大学 ， 凝聚态物理， 2008， 硕士

【摘要】 第一性原理（从头算方法）是量子力学的基本计算方法，大量用于物质或团簇（cluster）的结构、稳定性、导电性及光谱的理论研究，同时这些性质在量子化学中是研究内涵十分丰富的学科分支，对材料使用和新材料的研究应用方面占据着核心的位置。目前大型、高速电子计算机的应用，使得此理论研究的优越性越来越突出。理论研究也能很好地对实验进行验证、解释、补充甚至预言的作用。因此本文首先采用密度泛函方法研究了一氧化硅四聚体（Si₄O₄）和高压下正戊烷的结构、振动频率和光谱，得到Si₄O₄的两种稳定结构；也对实验中，正戊烷高压下相变点的大小进行了验证。与此同时，本文还用第一性原理方法对Sr₂YRuO₆有序双钙钛矿的结构和能带进行计算。在此基础上分析了该材料有序性存在的原因及Mott绝缘体特性。本论文主要包括以下四个部分：1、运用Gaussian03软件，通过量子化学HF，B3LYP和MP2方法，选用6-311G*基组对立方状、平面环状和折叠环状一氧化硅四聚体（Si₄O₄）的几何构型、稳定性、电子结构和振动频率等性质进行研究，部分结果与实验吻合，振动也与群论方法分析结果一致。研究表明：平面环状结构不是稳定结构，Si₄O₄存在T_d群对称性的立方状和D_2d群对称性的折叠环状两种结构，折叠环状结构能量更低，更稳定；两种结构均有微弱共价键，是绝缘体。2、本文在压强为0到17GPa范围内对正戊烷分子的结构、频率和光谱进行理论研究，得到在各个压强下正戊烷分子结构，频率以及红外和拉曼光谱。结构变化与实验吻合，但发现12．3GPa左右变化出现奇异。红外和拉曼光谱图中与实验结果较吻合，预言的相变压强点与实验也基本一致，说明这方面理论研究有一定的意义。理论上对几个特殊频率拉曼位移和频率比值加以分析，与实验吻合较好，对验证和预测相变也能提供一些依据。最后还得到正戊烷加压时，单偏转振动模式会先向全贯穿模式转换，12．3GPa后又会占据主导地位。这和实验上保持一致，是解释谱分裂及固固相变原因的重要依据。3、运用第一性原理方法对双钙钛矿Sr₂YRuO₆各种可能的无序结构和有序结构的能量进行计算，计算结果显示有序结构双钙钛矿具有最低能量，说明该结构最稳定，这一结果也与实验中样品的多晶样品X射线衍射图（XRD）中出现超晶格有序峰相符合。通过计算给出该材料的能带，计算结果显示该材料是Mott绝缘体，得到与实验结果相一致的结论。更多还原

【Abstract】 First Principle which is a method of calculation based on the principles of quantum mechanics is used widely in science, such as the structure, stability, electrical conductivity and spectral of material or clusters. And study of those properties of material and clusters is the most alive subject branch, with very abundant research connotation in the scientific and technological fields, and it is occupying central position in research application of the whole new material. Now the using of large and high-speed electronic computer make the advantage of the theory research more and more outstanding. The material experiments can be verified, interpreted, replenished and even predicted well by the theory. In this paper, the structure, the vibration frequency and spectrum of the silicon oxide tetramer （Si4O4） and the n-pentane under high pressure have been studied by use of the density function theory （DFT）B3LYP method, the conclusion is that there are two structures of Si4O4, and the phase change of n-pentane is verified with experiments. At the same time. First Principle is applied to study the structure and band of Sr₂YRuO₆. The material with order and characteristic of Mott insulators are analyzed basis on the calculation. The main contents are the following:1、The structure, vibrational frequencies, corresponding normal mode assignments, the infrared intensities and Raman activity of the silicon monoxide tetramer（Si₄O₄） in T_d symmetry and the isolated Si₄O₄ in D_2d symmetry are examined theoretically using the Gaussian03 set of quantum chemistry codes. All normal modes of buckled Si₄O₄ were ’Successfully obtained by a group theoretical analysis with the D_2d symmetry of the molecule. The conclusion is that Si₄O₄ possesses the cubane-like structure as C₄O₄ and Ge₄O₄. The planar Si₄O₄ may not be a stable structure. Comparing silicon monoxide tetramer with isolated buckled Si₄O₄, it shows that buckled Si₄O₄ can be observed easily because of it’s lower energy. the two molecules are insulator and held together in part by electrostatic forces.2、The structure, vibrational frequencies, the infrared intensities and Raman activity of n - pentane are studied theoretically under high pressure whose range is from 0 to 17 GPa. The structural changes are consistent with the experimental data, but the changes at 12.3 GPa is strange. The Infrared and Raman spectra of n - pentane is also in agreement with the experimental results. The pressures for phase transition occurring in experiment and theory are nearly the same. Several analysis in intensity ratios according to the experimental conclusion is useful for predicting phase transition. We also conclude that: n-pentane prefers to be an in-all-trans conformation in the pressure range of 3-12.3 GPa, and has end gauche conformers above 12.3 GPa, which is important to understand the solid to solid phase transition.3、The energies of all the possible structures of the double perovskite Sr₂YRuO₆ were calculated using First - principles method. The conclusion is that the energy of the double perovskite structure is lowest. It is consistent with the experimental result given by the X-ray diffraction data. There is an orderly XRD peaks in the experiment. We also computed the band of the double perovskite. It shows that Sr₂YRuO₆ is a Mott insulator material, this is also consistent with the experimental result.更多还原